The present invention relates to a process for removing H.sub.2 S and other sulfur species under anaerobic conditions from sour gases. In particular, the present invention relates to the microbiological treatment of sour natural gas to remove H.sub.2 S and other sulfur species, such as carbon disulfide, methyl mercaptan, ethyl mercaptan and dimethyl sulfide, from the natural gas stream, and recover elemental sulfur as a product of the process.
Natural gas reserves in the U.S. often contain hydrogen sulfide (H.sub.2 S) as a major contaminant. Hydrogen sulfide is an acid gas that is toxic and corrosive in the presence of water. A significant portion of total gas production does not meet pipeline standards and needs treatment to reduce the H.sub.2 S concentration to 1/4 grain per 100 standard cubic feet, or .ltoreq.4 ppm on a volume basis.
A commonly used commercial process for the removal of H.sub.2 S from the gas stream is the amine process, followed by the Claus process for sulfur recovery. In the amine process, the gas stream is contacted with the amine solvent to remove H.sub.2 S, then the amine solvent is heated to 90-150.degree. C. (194-302.degree. F.) to liberate H.sub.2 S and regenerate the solvent, which is recycled. Although the H.sub.2 S is removed from the natural gas stream, it still must be disposed of. Hydrogen sulfide generated during regeneration of the amine solvent can either be incinerated, which converts the hydrogen sulfide disposal problem into an air pollution problem due to the production of SO.sub.2, or treated by physicochemical methods such as the Claus process. In the Claus process, H.sub.2 S is fed into a reaction furnace, and the reaction gas is passed through a series of catalytic reactors to convert the H.sub.2 S into elemental sulfur. Although the Claus process produces a high quality elemental sulfur product, the process is often too expensive for small capacity plants (of less than 2 MM SCFd).
Several microbiological methods have been investigated for the treatment of gas streams containing sulfides. In one process, the anaerobic photosynthetic bacterium Chlorobium thiosulfatophilum is used to convert sulfides to sulfate. Cork, D. J. and Ma, S. "Acid-Gas Bioconversion Favors Sulfur Production", Biotech. and Bioeng. Symp. No. 12, 285-290 (1982).
In another process, which is the basis for a process known as Bio-SR, the Fe.sup.+2 formed during H.sub.2 S oxidation in accordance with Equation (1), is converted to Fe.sup.+3 by the bacterium Thiobacillus ferroxidans in accordance with Equation (2). EQU H.sub.2 S+Fe.sub.2 (SO.sub.4).sub.3 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 +S(Equation 1) EQU 2 FeSO.sub.4 +H.sub.2 SO.sub.4 +1/2O.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O (Equation 2)
A number of bacteria (called chemoautotrophic) use reduced sulfur compounds as a source of energy, CO.sub.2 or bicarbonate as a source of carbon, and NH.sub.4.sup.+ as a source of reduced nitrogen. Thiobacillus denitrificans is one such organism. One process for the desulfurization of gas using Thiobacillus denitrificans is disclosed in Sublette, U.S. Pat. No. 4,760,027. That patent describes a process wherein bacteria of the Thiobacillus genus convert sulfides to sulfates under aerobic conditions and at a controlled temperature of about 30.degree. C.
Most of the studies on H.sub.2 S removal have been performed under aerobic conditions and at H.sub.2 S concentrations of &lt;1000 ppm. Such methods, however, cannot be used for the direct removal of H.sub.2 S from sour natural gas because of the potential danger of explosion when methane and air are mixed.
The process of the present invention overcomes these limitations and problems of prior art H.sub.2 S removal processes because the process is carried out under anaerobic conditions. The process is known to be effective for treatment of inlet H.sub.2 S concentrations of up to 10,000 ppm (1%), at a pressure of 1,000 psi and at temperatures common to those required by the gas industry (e.g. 140.degree. F., 60.degree. C.). In addition, the process of the present invention reduces CO.sub.2 levels of from 5% to 10% down to 2%.
In earlier research, ARCTECH developed a microbial consortium, SSII, from ARCTECH'S Microbial Culture collection (AMCC) to reduce the H.sub.2 S concentrations of up to 10,000 ppm to pipeline specifications of .ltoreq.4 ppm. The biological and physiological characteristics of this consortium and technical feasibility of the consortium to mitigate 1% H.sub.2 S to .ltoreq.4 ppm are the subject of a separate patent application. The information from laboratory scale bioreactor experiments using SSII served as the basis for further research, which is the subject of the present invention. The preliminary data from the laboratory scale bioreactor experiments was presented at the 1992 GRI Liquid Redox Sulfur Recovery Conference, Austin, Tex. on Oct. 4-6, 1992, by K. C. Srivastava, and entitled "Biological Removal of H.sub.2 S From Sour Natural Gas", which paper is hereby incorporated by reference in its entirety. The results from this preliminary work provided the experimental proof of the concept of biological H.sub.2 S removal under anaerobic conditions on bench scale. Nevertheless, additional scaled-up processing information was necessary for delineating the process parameters.